(a) Field of the Invention
The present invention relates to a laser crystallization device and a laser crystallization method.
(b) Description of the Related Art
Both liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays can be fabricated to be thin and lightweight. These types of flat panel display devices are commonly used as displays in mobile electronic devices, and their application coverage extends to large-scale display devices. As the necessity for display devices requiring high speed operational characteristics emerges, research on such a display device is actively ongoing.
To satisfy the high speed operational characteristics of a display device, the channel region of a thin film transistor (TFT) must be formed by using a material with a greater electrical mobility than that of amorphous silicon, whose electrical mobility is only about 0.1 cm/Vsec to 0.5 cm/Vsec. Polycrystalline silicon has an electrical mobility of about 100 cm/Vsec to 300 cm/Vsec, thereby enabling a much faster operational speed compared with amorphous silicon.
Excimer laser annealing (ELA) is a conventional method for forming polycrystalline silicon. In the ELA crystallization method, an amorphous silicon thin film deposited on a glass substrate is irradiated by a high energy pulsed laser beam. Upon absorbing the light energy from the laser beam, the amorphous silicon thin film melts, and then re-solidifies so as to be crystallized. An advantage of this method is that the glass substrate is not damaged by the heat.
In this laser crystallization method, the spatial energy distribution cross-section, or profile, of the output laser beam, which typically has a Gaussian distribution, is changed by using an optical system. The output beam shape is transformed into a rectangle with a very large aspect ratio. To improve uniformity and productivity of crystallization, the output energy distribution is transformed into a “top-hat” beam profile, which is as nearly uniform as possible over the entire focused rectangular beam, with sharp edges.
During the laser annealing process, if a persistent inhomogeneity appears in the laser beam profile, a corresponding linear pattern may be displayed on the silicon target along the scan direction of the laser. The linear pattern may be subsequently frozen into the polycrystalline silicon.
At this time, if the laser beam profile as the linear pattern may have an error portion, a linear error portion may be generated along a scan direction of the laser.
To prevent the linear error portion, a method of vibrating the focused laser beam along its long axis, perpendicular to the scan direction, has been attempted when forming the polycrystalline silicon thin film. By irradiating the vibrating laser beam onto the amorphous silicon thin film, however, there is a limitation in removing a defect of a dot pattern. This dot pattern inhomogeneity in the laser beam profile may cause undesirable nonuniformities in the polycrystalline silicon.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.